You Hear It First

We often think about what the building code requires but not about the building occupants where we install a fire alarm system. In a hotel, for example, a large number of people are in unfamiliar surroundings. This fact should be considered when designing or installing a fire system’s notification appliance layout and also how the system is programmed. The transient population must be notified of a fire alarm condition early, so they have time to evacuate.

The notification system design is especially important when renovating existing hotels. The typical approach is to replace all existing equipment, including the notification appliances, with new appliances in the same location as the old. If you fall into this trap, you certainly will fail the audibility test required by the National Fire Alarm Code.

NFPA 72-2007 requires notification appliances to operate in the “public operating mode.” This is defined as “Audible or visible signaling to occupants or inhabitants of the area protected by the fire alarm system.” Essentially, this means we must notify all of the occupants of an alarm condition. The code provides measurable sound level requirements that are enforceable by the fire official. First, you must establish the ambient sound level in the building where you are upgrading the fire alarm system. Typically, older hotel buildings will have a mix of construction types, some of which will provide more sound absorption than others. The average ambient sound levels are in the range of 35 to 55 dBA. These measurements are made with a sound level meter using the A-weighted scale (dBA).

The code requires audible public mode signals be heard clearly by all occupants, and the signals must “have a sound level at least 15 dB above the average ambient sound level or 5 dB above the maximum sound level having a duration of at least 60 seconds, whichever is greater, measured 5 feet above the floor in the area required to be served by the system.”

In our hotel example, the sound level measured in all occupiable areas must be at least 50 to 70 dBA. Obviously, some common sense must be used, and the requirements do not apply to small storage areas or janitorial closets.

These sound levels cannot be obtained in the one-for-one replacement example outlined above. In virtually all of the early fire alarm system designs, the audible notification was assumed adequate if you could hear it in the hallways. The requirements today mandate that the alarm signal be heard everywhere people congregate or sleep.

The sleeping areas actually have more specific requirements. The code mandates audible appliances installed to provide signals for sleeping areas must have a sound level of at least 15 dB above the average ambient sound level or 5 dB above the maximum sound level, having a duration of at least 60 seconds or a sound level of at least 75 dBA, whichever is greater, measured at the pillow level (generally 2–3 feet above the floor) in the area required to be served by the system. The key point is that 75 dBA is the required sound level. Additionally, if there is any barrier, such as a door, curtain or retractable partition between the notification appliance and the pillow, the sound pressure level must be measured the same way.

You won’t achieve these mandated sound levels using hallway notification appliances. Often, designers add smaller horns in the sleeping areas to ensure compliance with the more stringent sound level requirements.

If the original design did not have adequate notification appliances to meet current code requirements, additional appliances will be needed in all sleeping rooms and probably also in the meeting and assembly areas. If the owner is upgrading a portion of the fire alarm system only on renovated floors, another issue may present itself when you cannot match the existing appliances.

As stated in the annex of the code, all audible notification appliances within a building do not have be of the same type. “However, a mixture of different types of audible notification appliances within a space is not the desired method. Audible notification appliances that convey a similar audible signal are preferred. For example, a space that uses mechanical horns and bells might not be desirable. A space that is provided with mechanical horns and electronic horns with similar audible signal output is preferred.

However, the cost of replacing all existing appliances to match new appliances can impose substantial economic impact where other methods can be used to avoid occupant confusion of signals and signal content. Examples of other methods used to avoid confusion include, but are not limited to, training of occupants, signage, consistent use of temporal code signal pattern and fire drills.”

The professional contractor understands that alarm notification of occupants is one of the most important aspects of any fire alarm system and will ensure that the proper number of appliances is installed to meet the requirements of NFPA 72.

MOORE, a licensed fire protection engineer, frequent speaker and an expert in the life safety field, is a co-editor of the current National Fire Alarm Code Handbook. Moore is a principal with Hughes Associates Inc. at the Warwick, R.I., office.